The human spine includes a series of vertebras nominally divided up into sections by form and function. Starting from the top of the spine, the sections include the cervical vertebras, the thoracic vertebras, the lumbar vertebras, the sacral, and the coccyx. The cervical (neck) vertebras, for example, include the top seven vertebras that support the head. The topmost vertebra is C1 (or C-1), followed by C2. The lowest vertebra of the cervical vertebras is C7.
Sometimes, the joint between the vertebras can become unstable, for example, due to trauma, rheumatoid arthritis, or a tumor. The instability can be treated by techniques known as spinal fixation in which surgical implants mechanically immobilize selected vertebras, and allow the treated vertebras to fuse and to stabilize.